Curable resins such as phenolic resins, unsaturated polyester resins and epoxy resins are excellent in heat resistance, mechanical strength and dimensional accuracy and are thus widely used in various fields. Meanwhile, formed articles formed from curable resins such as epoxy resins show the problem of high brittleness owing to their low fracture toughness.
Adding a rubber component in an epoxy resin composition is known as a method for enhancing toughness by adding a modifier to an epoxy resin.
Examples of such a method of adding a rubber component include a method of adding a reactive liquid rubber (e.g. CTBN) or a nitrile rubber and a method of mixing a core-shell polymer with an epoxy resin. However, reactive liquid rubbers undergo a process in which they are temporarily dissolved in epoxy resins and then phase separation occurs during curing. Thus, the morphology of a cured product formed from such a reactive liquid rubber varies depending on the type of epoxy resin used and the particular curing conditions, which results in failing to achieve a desired modifying effect and in a problem with reproducibility in quality. It is also known that part of the rubber component remained dissolved in the cured epoxy resin phase lowers the elastic modulus and the glass-transition temperature of the cured product to cause problems including a reduction in quality of the final epoxy resin product. Moreover, the method of adding a core-shell polymer to an epoxy resin allows to suppress reduction of glass-transition temperature but has the following problem. Commercially available core-shell polymers are provided as aggregates (agglomerates) of primary particles, for example, in a powder form with a particle size of several tens to several hundreds of micrometers. To mix such a core-shell polymer with an epoxy resin, the core-shell polymer needs to be finely divided so that the particle size is less than 10 μm and, furthermore, they require to be thoroughly mixed by a kneading machine, such as a stirrer with heating means at 50° C. to 200° C., a high shear stirrer, a heating roller, an intermixer, a kneader, or a three-roll mill; otherwise, there is the problem that the mixed core-shell polymer easily precipitates or floats up and is separated.
Meanwhile, Patent Literature 1 discloses a method for producing a resin composition containing well-dispersed rubber polymer particles with a decreased amount of impurities. The method includes mixing an aqueous latex of rubber polymer particles with an organic solvent which is partially soluble in water; contacting the resulting mixture with water to form aggregates of rubber polymer particles; separating the aqueous phase from the mixture of the aggregates and the aqueous phase to recover the aggregates of rubber polymer particles with a decreased amount of impurities; adding an organic solvent to the aggregates to prepare a dispersion; mixing the dispersion with a reactive group-containing polymerizable organic compound such as an epoxy resin; and distilling off volatile components. Such a method, however, still has room for further improvement in achieving good dispersibility in a curable resin containing a thermoplastic resin, as well as in achieving good dispersibility in and toughness of a cured product formed from a curable resin composition.